x:Metric: say which node wrote each sample

Each node stores histograms as running totals since it started. A sample
didn't say which node wrote it (the node was only in the id's low bits),
so a reader couldn't diff totals per node, and the console diffed across
nodes: on the three-node production cluster the delivery attempt time
read 14.7 s over the last hour against 0.7 s from the nodes' own figures.

x:Metric/get now returns nodeId alongside timestamp, both from the id.
The telemetry suite checks every sample carries it.

crates/common/src/telemetry/metrics/store.rs

@@ -104,6 +104,12 @@ impl StoredMetric {
     pub fn timestamp(&self) -> u64 {
         SnowflakeIdGenerator::to_timestamp(self.id)
     }
+
+    /// The node that wrote the sample. Histogram totals are per node, so a
+    /// reader diffs them per node.
+    pub fn node_id(&self) -> u64 {
+        SnowflakeIdGenerator::to_node_id(self.id)
+    }
 }
 
 /// What the node wrote last, so counters and histograms are written as